This invention relates to a liquid crystal device of the torsion effect type formed by employing a nematic liquid crystal material having a positive dielectric anisotropy (hereinafter referred to merely as "Np-liquid crystal").
In a liquid crystal device of the torsion effect type, the change of orientation of the Np-liquid crystal molecules caused under application of an electric field, a magnetic field, a ultrasonic wave or the like is utilized for light modulation. When an electric field is applied, there is employed a liquid crystal device of the torsion effect type formed by applying a transparent conductive coating on one surface of a support such as glass sheet to form an electrode plate, arranging a pair of so formed electrode plates so that the coated electrode surfaces confront each other (the distance between the two electrode plates is usually 1 to 100 .mu.), to thereby form a cell, and filling an Np-liquid crystal material into the cell according to customary methods such as a pressure injection method comprising charging the liquid crystal material into the cell under pressure by using an injector, or a vacuum injection method comprising maintaining the inside of the cell under vacuum and then charging the liquid crystal into the cell under atmospheric pressure. In this case, the electrode surface is pre-treated so that the Np-liquid crystal molecules are oriented in a certain direction, and a pair of the so treated electrode plates are arranged so that the orientation direction of the Np-liquid crystal molecules in one electrode surface crosses vertically the orientation direction of the Np-liquid crystal molecules in the other electrode surface. In the so obtained liquid crystal device, the Np-liquid crystal molecules are oriented in such state that the direction of the long axis of the molecule is in parallel to the electrode surface and between the electrode surfaces the molecules are continuously twisted by 90.degree.. Since the pitch of this torsion is much larger than the wavelength of light, plane of polarization of linear polarized light perpendicular to the electrode plate is rotated by 90.degree. while it is passing through this liquid crystal device of the torsion effect type. Accordingly, this liquid crystal device of the torsion effect type shields light when it is disposed between two polarizers arranged so that light oscillating faces of the polarizers are in parallel to each other, and when the liquid crystal device is disposed between two polarizers arranged so that light oscillating faces of the polarizers cross vertically each other, it allows transmission of light. When a voltage is applied to this liquid crystal device of the torsion effect type, in response to the applied voltage the long axis direction of Np-liquid crystal molecules is inclined to the electric field direction, and at a voltage exceeding a certain limit the Np-liquid crystal molecules are arranged so that the long axis direction is substantially in parallel to the electric field direction. In this state, contrary to the case of no application of voltages, the Np-liquid crystal device allows transmission of light when it is disposed between parallelly arranged polarizers but it shields light when it is disposed between polarizers arranged vertically to each other. Accordingly, when such liquid crystal device of the torsion effect type is inserted between two polarizers, the liquid crystal device changes its state from the light shielding state to the light transmitting state or from the light transmitting state to the light shielding state in response to the applied voltage, and this change, namely light modulation, can be utilized for display or the like.
In preparing liquid crystal devices of the torsion effect type (hereinafter referred to simply as "liquid crystal device"), it is important that electrode surfaces should be treated so that Np-liquid crystal molecules are oriented in a certain direction. As the conventional electrode surface treatment method, there can be mentioned a method comprising polishing the electrode surface in a certain direction directly with dry cloth, paper, rubber or the like. According to this conventional method, however, it is impossible to orient liquid crystal molecules sufficiently and uniformly so that the long axes of the liquid crystal molecules are in parallel to the electrode surface. Accordingly, in the resulting liquid crystal device, orientation of the liquid crystal molecules is insufficient and non-uniform, which results in the following defects of electro-optical characteristics:
1. Operation voltage is high. PA1 2. A liquid crystal device having an area exceeding 1 cm.sup.2 fails to show a uniform electro-optical response throughout the liquid crystal device. PA1 3. Differences in the operation voltage of about 1.5 to 2 times occur throughout the same device. PA1 4. It is difficult to obtain a good contrast ratio.
The conventional methods for treating the electrode surface as mentioned above are disadvantageous in that the polishing of the electrode surface needs a high pressure of 10 to 50 kg/cm.sup.2 so that it is difficult to prepare stabilized devices and in the mass production differences in the operation voltage of 1.5 to 3.0 times occur among the devices.
As the electrode surface treatment means overcoming these defects, there has been proposed a method comprising forming on the electrode surface a coating of an organic polymeric material such as a silicone resin, an epoxy resin, an acrylic resin and a phenol resin and polishing this coating with cloth, paper or the like (U.S. patent application Ser. No. 485,036 filed on July 1, 1974). According to our experiments, however, it is impossible to obtain liquid crystal devices having satisfactory electro-optical characteristics, and when mass production is conducted according to this method, deviation of electro-optical characteristics is very great among lots. The above U.S. patent application discloses a liquid crystal device including electrode plates formed by coating a cellulose resin on the electrode surface and lightly polishing the coating in one direction with a brush, paper, cloth or the like, and it is taught that this device has a good orientation in Np-liquid crystal molecules and in turn, good electro-optical characteristics, and that no deviation of electro-optical characteristics is brought about among lots in the case of mass production. However, a cellulose resin coating is defective in that its heat stability and chemical resistance are poor. More specifically, the softening point of a cellulose resin is low and therefore, when an electrode having a surface coated with a cellulose resin is heated at a temperature higher than 150.degree.C., the resin coating is softened and the effect of the polishing treatment made on the resin coating is lost. Especially, nitrocellulose is inferior in the heat stability and it is decomposed when it is heated at a temperature higher than 140.degree.C. Further, the resistance of a cellulose resin to organic solvents such as alcohols, ketones, esters and aromatic hydrocarbons is insufficient, and it is easily swollen upon contact with these organic solvents and easily dissolved therein.
When a cell for an Np-liquid crystal is prepared by using a pair of electrode plates, there is generally adopted a method comprising coating a thin band of a high molecular adhesive such as an epoxy resin, a melamine resin, a phenol resin, an acrylic resin or a urethane resin on the peripheral edge portion of the electrode surface of one of pre-treated electrode plates except for an opening for charging of an Np-liquid crystal material according to the screen printing method, and bonding the other pre-treated electrode plate to the adhesive coated electrode plate so that both the electrode surfaces confront each other and the treatment direction on the electrode surface of one electrode plate crosses vertically the treatment direction on the electrode surface of the other electrode plate. In the so prepared cell, a thin band of the polymeric adhesive formed on the peripheral edge portion acts not only as a spacer for keeping a certain distance between the two electrode plates but also as a sealing agent for bonding the two electrode plates. In this case, use of a high temperature curing adhesive is preferred. The reasons are as follows:
When a high temperature curing adhesive is employed, a reaction to high molecules is promoted completely by heating and a pair of electrode plates are tightly bonded and sealed. Simultaneously, no unreacted low molecular weight by-product is left because of completion of the reaction to high molecules. The presence of such low molecular weight by-product is considered to be one of causes of degradation of an Np-liquid crystal material charged in the so formed cell, because such by-product reacts with molecules of the Np-liquid crystal.
As is apparent from the foregoing, when a cell is prepared by using a pair of polish-treated electrode plates having an electrode surface coated with a cellulose resin, the high molecular adhesive of a high temperature curing type cannot be used, but a low temperature curing or room temperature curing adhesive is reluctantly used. As pointed above, a high molecular adhesive is coated on the electrode surface by the screen printing method. In this coating method, an organic solvent is added to the high molecular adhesive to adjust its viscosity to one suitable for coating. As stated above, a cellulose resin is insufficient in the chemical resistance, and therefore, in the case of electrode plates having a cellulose resin coating, the kind of the solvent to be used for coating of the high molecular adhesive should naturally be limited.